Manufacturers have many options for creating plastic products. One that is popular for creating hollow plastic products is rotational molding, also called rotomolding. Rotomolding is used across industries to make a variety of products, including diesel tanks, seed hoppers, recreational vehicle components, furniture, and more. Rotomolding is a preferred process for many because of its benefits, but we would be remiss not to mention its limitations. In this article, we will explain how rotomolding works and shed light on its advantages and limitations.
How Rotomolding Works
The rotomolding process is a three-stage process that uses heat, low-pressure, and bi-axial rotation to create plastic parts inside a mold. Bi-axial means it is rotating on two axes simultaneously. It should be noted that it is not using centrifugal force to whip material around inside the mold.
Before the process can begin, molds must be built. At GVL Poly, steel molds are designed and built in-house. If an aluminum mold is required, we have partners we trust to build consistently high-quality aluminum molds.
- Fill – The first step is to fill the mold with a measured amount of powdered resin. Several resins can be used, the most popular being the polyethylene family (LLDPE, PEX, LDPE, HDPE). Other resins are polypropylene, polyvinyl chloride, nylon, and polycarbonate. We can custom dry blend as well to offer even more options.
- Sinter and Densification – The mold slowly rotates bi-axially and enters the oven, where the powdered resin slowly reaches a temperature just at its melting point. When the interior mold surface reaches a designated temperature, the plastic powder particles adhere to the surface, and the heat causes the particles to bond at their contact points without completely melting them. This forms a skin of simi-fused particles against the mold surface.
- Densification follows. The temperature is either increased or maintained for longer, allowing the resin to become molten. As it densifies, it creates a uniform solid layer that conforms precisely to the mold shape.
- Cooling – Cooling is paramount to achieving the correct material properties of the product. Circulating air around the mold dissipates the heat from the plastic in a controlled manner so the part cools uniformly. Cooling time depends on the size of the part and its wall thickness. Once the part cools, it is removed from the mold and moved on to any secondary processes that may be needed.
Benefits of Rotomolding
It is possible to use other methods for forming hollow parts, such as blow molding, injection molding, or thermoforming; however, rotational molding is often a preferred choice for many products because of its benefits.
- No Seams Or Joints – One of the primary benefits of rotomolding is that the part is a single piece with no seams or joints. Seams and joints can impact the aesthetics of a part, but joints and corners also tend to be weaker and could result in breakage under stress. Injection molding and thermoforming cannot make hollow parts without joining two halves together.
- Consistent Wall Thickness – The bi-axial rotation and controlled heating and cooling create parts with uniform wall thickness. Uneven wall thickness is a common issue with blow molding.
- High Strength – The strength-to-weight ratio for rotomolded products is exceptional. The uniform wall thickness, reinforced points, and thicker corners enhance the product’s strength, making rotomolding a perfect choice for products requiring durability and lighter weight.
- Design Flexibility – Rotomolding allows for great flexibility in the design of hollow parts, allowing the formation of both large and small products with complex shapes and intricate details. Double-wall containers can be molded without secondary operations. Blow molding doesn’t offer the same level of design flexibility as rotomolding.
- Cost-effective – Rotomolding has lower production costs compared to other methods like blow molding or injection molding, as tooling costs and energy consumption tend to be much lower.
Limitations of Roto-Molding
While rotomolding is an exceptional manufacturing method for hollow plastic parts, its limitations may require a different method in some cases.
- Long Cycle Times – Rotomolding traditionally has much longer cycle times than other methods. However, some of our machines employ Smart Molding Technology that uses electric resistance heaters directly mounted on the mold rather than using an oven. This creates increased efficiency, shorter cycle times, less material usage, and a higher quality product.
- Material Limits – While there are many resins and resin combinations to choose from, not all resins are available in powdered form.
- Low Volume – Because each mold typically holds one product and the cycle times are long, rotomolding is only suited to lower volumes. High-volume products are better suited to a different manufacturing method.
- Larger Tolerances – If very tight tolerances are needed, look for another method. While rotomolding can create parts with uniform walls, its tolerances tend to be larger.
If you are considering rotomolding for your product and aren’t sure where to start, talk to our experts to see if it is right for you. With decades of experience taking projects from concept to finished product, our team of experienced machine operators, engineers, designers, and tool builders will ensure your product meets your expectations. Contact us to start the conversation.